1. Field of the Invention
This invention relates in general to inertial navigators and in particular wherein it is desired to determine and correct for azimuth-axis drift rate. The correction for azimuth-axis drift rate provides improved directional accuracy and improved navigational performance.
2. The Description of the Prior Art
It is well known by those skilled in the art that determination of azimuth-axis drift rate is illusive, because, on a short-time basis, the manifestations of such a possible error source in inertial navigators are small and not readily observable among the larger manifestations of other possible error sources.
The prior art has attempted to utilize external navigation aids (such as the Global Positioning System, GPS) and an optimal estimation filter to correct the system during flight. We use the phrase "correct the system" to denote the alignment and initialization of the system, and the calibration (correction) of the system error sources.
However, this technique suffers from the fact that the system model of the filter does not allow the azimuth-axis drift rate to be observable in the presence of other error sources in the system within the relatively short-time system updates produced by the filter.
An added complication is that the longitudinal (azimuth) axis of an inertial navigator is coupled to the lateral (North and East) axes thru the Foucault (24-hour) pendulum effect which is typically not included in the system model of the filter.